The invention relates to a piston of an internal combustion engine which is produced from steel and has a top part and a bottom part, the joining webs of which are integrally bonded in the region of a contact geometry by means of a friction weld to form a structural unit. For cooling, at least one cooling duct permeated by a coolant is integrated in the piston.
Zones of pistons for internal combustion engines which are subjected to high thermal loading, for example the combustion chamber cavity in the piston head or the region of the first groove, require effective cooling. For this purpose, it is known to integrate a cooling duct in the piston. A coolant, preferably the lubricating oil of the internal combustion engine, circulates through the cooling duct, which is preferably in the form of a hollow space. The efficiency of the piston cooling is governed, in particular, by the exchanged volume of the coolant in the cooling duct.
U.S. Pat. No. 6,155,157 discloses a cooling duct piston comprising two components which can be produced separately from one another and are joined together integrally via a friction welding process. The cooling duct provided is a rotationally symmetrical annular duct which is spaced apart circumferentially from the annular field of the piston and is permeated by a coolant, in particular the lubricating oil of the internal combustion engine, via an immovably positioned injection nozzle.
DE 39 06 582 C1 discloses a multi-part piston, the individual parts of which are connected by means of a friction weld so as to obtain an integral bond and a structural unit. In this process, the components are rubbed against one another under compressive loading, preferably by a rotary movement, until the contact geometry or the joining zone has reached the temperature suitable for welding the two components. The heating by rubbing is followed by a compression operation, in which the soft or molten regions of the components are torn open, plastically deformed and pressed onto one another. A weld bead which emerges from the contact geometry is thereby formed on both sides of the joining web.
Furthermore, there are pistons with a small compression height in which relatively short joining webs are established between the top part and the bottom part of the piston owing to the overall height of the piston. Owing to the confined installation space associated with this in the region of the contact geometry, there is the risk that the inlet and outlet openings of the cooling duct, which are formed as a bore, will taper off owing to the weld bead from the friction welding process. As a result, the exchange of coolant is reduced, and this is associated with a direct disadvantageous influence on the cooling of the piston.
It would be desirable to provide a cooling duct piston with an improved cooling action, in which the residence time of the coolant in the cooling duct is extended.
In one aspect, for the introduction of the coolant into the cooling duct, the inlet opening includes a flow pipe which is inserted in the bottom part. Here, the opening of the flow pipe, which protrudes from a cooling duct base with an excess length, is positioned above a contact geometry and its associated weld bead of the friction welding process.
The flow pipe is arranged in the contact geometry arising between the top part and the bottom part in such a way that it protrudes from a cooling duct base with an excess length. The extent of the excess length is selected in such a way that an opening of the flow pipe is positioned above a weld bead which arises by means of a friction weld, from a friction welding process. The exchange of coolant, which is decisive for the efficiency of the piston cooling, is therefore retained in an unrestricted manner, since no cross-sectional tapering, which has a negative effect on the cooling action of the piston, can be established owing to a weld bead in the region of the inlet opening of the cooling duct.
During an upward stroke of the piston, the coolant located in the cooling duct is pressed onto the cooling duct base owing to the acceleration of the piston. In this case, the installed position of the flow pipe prevents the return flow or exit of the coolant via the inlet opening of the cooling duct and ensures optimum filling of the cooling duct extending over a crank angle of 360°. At the same time, a disadvantageous influence on the flow owing to coolant exiting at the inlet opening, which leads to the foaming of oil, is prevented.
The reversal of the piston movement, which takes place rapidly in the top dead center of the piston, means that the coolant or lubricant located on the cooling duct base is hurled owing to the inertia action over a large area against the outer surface of the cooling duct which is formed, in particular, by a bottom side of the piston head. In combination with the alternating movements of the piston, the coolant acting on the entire cooling duct brings about optimization of the flow of coolant and also improved exploitation of the shaker action.
An increased residence time of the coolant (cooling oil) in the cooling duct and a reinforced shaker effect bring about an improved and desired dissipation of heat from the piston regions which are subjected to high thermal loading and are to be cooled.
The coolant which moves over the entire cooling duct ensures that the piston zones subjected to high thermal loading are effectively cooled without there being the risk of combustion of the coolant.
According to one aspect of the invention, a clearance “x” between the opening of the flow pipe and the cooling duct base exceeds a dimension “y”, which defines a fill level of the coolant in an upward movement of the piston in the cooling duct. The differential dimension or clearance which is established ensures that disadvantageous return flow and an associated influence on the jet of the coolant in the region of the inlet opening is prevented irrespective of the direction in which the piston moves. Before the friction welding, the dimensionally stable flow pipe is arranged in a receiving bore of the bottom part of the piston in such a way that the clearance “x” is established after the latter is joined together with the top part of the piston.
Long-term fixing of the flow pipe takes place via a weld bead which is formed during the friction welding, by means of which the top part of the piston and the bottom part of the piston are integrally bonded to one another. A positively locking fastening can take place between the weld beads and the flow pipe in order to improve the fixing. For this purpose, the outer contour of the flow pipe has a recess intended for receiving the weld seam. An annular groove or a partially concave outer contour is preferably provided in order to constitute a recess which can be implemented at low cost.
In order to achieve optimum adaptation and positional fixing of the separate flow pipe in the piston, use is made of a pipe produced from steel. A material which is similar to the piston material in particular with respect to the coefficient of thermal expansion is selected for the flow pipe, and therefore reliable, play-free fastening is also ensured in the operating state of the internal combustion engine.
The flow pipe, which forms the inlet opening, is inserted in the bottom part of the piston and fixed via at least one weld bead and extends in the direction of the ends of the piston skirt. It is thereby possible to advantageously reduce the distance between the injection nozzle and the flow pipe, as a result of which it is possible to improve the effectiveness and the efficiency of the application of coolant via the injection nozzle into the flow pipe.
A further advantageous refinement provides for the flow pipe to be provided with a curved outlet opening on the cooling duct side. The flow pipe has two outlets, which point in opposite directions and are arranged in a Y-shape with respect to one another. This measure takes into account the rapid reversal of the piston movement at the bottom and top dead center and the associated transfer of the coolant between the cooling duct base and the outer surface of the cooling duct, which, if the flow pipe is arranged perpendicularly or parallel to a lateral surface of the piston, can have the effect that a sub-quantity of the coolant acts upon the flow pipe during the shaker movement. The flow pipe arched at the end with an outlet which is oriented preferably at an angle to the direction in which the piston moves or to a longitudinal axis of the piston prevents a disadvantageous inlet of coolant irrespective of the direction in which the piston moves. Furthermore, a flow pipe with two openings pointing in opposite directions adopts the function of a flow divider, which splits the flow of coolant in a targeted manner into two directions so that the latter acts uniformly on the annularly extending cooling duct.
In addition, the inlet opening can be transferred to a piston with a split cooling duct. For this purpose, each cooling duct half or each cooling duct segment is assigned an inlet opening and an outlet opening, each inlet opening preferably being assigned a separate injection nozzle. This design makes it possible for a relatively large flow of coolant to act on the cooling duct. Depending on the position of these openings, the coolant can flow through the cooling duct segments in cocurrent or in countercurrent.
A process for producing the piston, includes the following steps: firstly, a flow pipe is inserted into a receiving bore of the bottom part of the piston, where attention is paid to an installed position which ensures that the opening or the outlets of the flow pipe is or are arranged above a contact geometry. The top part and the bottom part of the piston are then assembled, before these components are joined together integrally by means of a friction weld. The weld beads that form in this process in the region of the contact geometry at least locally bring about nonpositive and/or positive fixing of the flow pipe.